Pressurized-water nuclear reactors generally comprise a vessel enclosing the core of the reactor which is immersed in the pressurized cooling water of the reactor.
The vessel of the reactor of overall cylindrical shape comprises a head of hemispherical shape which may be attached onto its upper part.
The head is pierced with openings in the region of each of which there is fastened, by welding, a tubular penetration piece constituting an adapter providing the passage for and controlling the movement of an extension of a control cluster for the reactivity of the core or a penetration passage for means for measurement inside the core, such as a thermocouple column.
To the end parts of each of the adapters, there are fastened mechanisms for controlling movement of the control clusters for the reactivity of the core.
Inside each of the tubular penetrations of the vessel head there is fastened, in a position which is coaxial with respect to the tubular penetration piece and with a certain radial clearance, a thermal sleeve which comprises a diametrically flared part coming to rest on a diametrically flared bearing surface located at the upper part of the bore of the tubular penetration piece and which is mounted to rotate freely inside the penetration piece.
The extensions of the rods for controlling the reactivity of the nuclear reactor and the thermocouple columns pass through the vessel head inside thermal sleeves which are themselves arranged coaxially inside adapters for the control rods or more generally inside tubular penetration pieces of the head.
In order to increase the reliability and operational safety of nuclear reactors and to extend the durability of these reactors, plant operators are led to carry out more and more numerous inspections of the various elements making up the nuclear reactor.
In particular, it may be necessary to inspect the state of the penetration pieces of the head of the vessel in order to be sure of the integrity of these pieces after the reactor has been in operation for a certain time, in particular in the zone where these tubular pieces are welded to the head. As a function of the result of the inspection, detected faults may be repaired, by excavating the internal surface of the adapter in the zone having a fault and by building back up the cavity produced by excavation.
In French Patent Application No. 92 02405 filed by FRAMATOME and Electricite de France on Feb. 28, 1992, it was proposed to carry out an inspection of the internal surface of the adapter using ultrasound or eddy currents, through a slit machined in the thermal sleeve in its longitudinal direction. It was also proposed to carry out excavation by machining through the slit, when a crack is detected on the internal surface of the adapter.
In French Patent Application No. 92 02405, filed on Aug. 6, 1992 by FRAMATOME, it was also proposed to excavate zones having faults by machining the internal surface of the adapter to a slit made in the thermal sleeve using a jet of pressurized abrasive liquid.
These methods make it possible to inspect and, if necessary, repair faults without dismantling the thermal sleeve arranged inside the adapter.
However, such methods do not make it possible perfectly to characterise the faults detected and in particular to determine whether these faults extend deeply into the wall of the adapter, and, for this reason, whether they are likely to pass through this wall or to develop so as to become penetrant.
By using the methods according to the prior art, one may be led to carry out repairs with excavation and building back up, which are not strictly necessary for the safety of the nuclear reactor.
Neither do the inspections carried out make it possible to determine very precisely the location, extent and geometric shape of the faults detected.